U.S. patent application number 13/718514 was filed with the patent office on 2013-06-20 for optical scanning apparatus and image forming device.
The applicant listed for this patent is Nobuaki KUBO, Masayuki MURANAKA, Kohji SAKAI, Toshiaki TOKITA, Naoto WATANABE. Invention is credited to Nobuaki KUBO, Masayuki MURANAKA, Kohji SAKAI, Toshiaki TOKITA, Naoto WATANABE.
Application Number | 20130155166 13/718514 |
Document ID | / |
Family ID | 48586862 |
Filed Date | 2013-06-20 |
United States Patent
Application |
20130155166 |
Kind Code |
A1 |
WATANABE; Naoto ; et
al. |
June 20, 2013 |
OPTICAL SCANNING APPARATUS AND IMAGE FORMING DEVICE
Abstract
An optical scanning apparatus includes a light source, an
optical deflector having a rotary polygon mirror to deflect a light
beam from the light source, a scanning optical system configured to
focus the light beam deflected by the optical deflector on a target
surface, a sync detecting sensor configured to determine a write
start timing on the target surface, and a processing unit
configured to correct detection data of the sync detecting sensor
based on a measured value of a time needed for one revolution of
the rotary polygon mirror.
Inventors: |
WATANABE; Naoto; (Kanagawa,
JP) ; TOKITA; Toshiaki; (Kanagawa, JP) ;
SAKAI; Kohji; (Tokyo, JP) ; KUBO; Nobuaki;
(Tokyo, JP) ; MURANAKA; Masayuki; (Kanagawa,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
WATANABE; Naoto
TOKITA; Toshiaki
SAKAI; Kohji
KUBO; Nobuaki
MURANAKA; Masayuki |
Kanagawa
Kanagawa
Tokyo
Tokyo
Kanagawa |
|
JP
JP
JP
JP
JP |
|
|
Family ID: |
48586862 |
Appl. No.: |
13/718514 |
Filed: |
December 18, 2012 |
Current U.S.
Class: |
347/224 ;
359/207.8 |
Current CPC
Class: |
G02B 26/123 20130101;
G03G 15/043 20130101; B41J 2/473 20130101; G03G 15/505 20130101;
G02B 26/127 20130101; B41J 2/471 20130101; G02B 26/129
20130101 |
Class at
Publication: |
347/224 ;
359/207.8 |
International
Class: |
B41J 2/435 20060101
B41J002/435; G02B 26/12 20060101 G02B026/12 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 19, 2011 |
JP |
2011-277161 |
Feb 7, 2012 |
JP |
2012-023572 |
Dec 4, 2012 |
JP |
2012-265706 |
Claims
1. An optical scanning apparatus, comprising: a light source; an
optical deflector having a rotary polygon mirror to deflect a light
beam from the light source; a scanning optical system configured to
focus the light beam deflected by the optical deflector on a target
surface to be scanned; a sync detecting sensor configured to
determine a write start timing on the target surface; and a
processing unit configured to correct detection data of the sync
detecting sensor based on a measured value of a time needed for one
revolution of the polygon mirror.
2. The optical scanning apparatus according to claim 1, wherein the
processing unit is configured to determine a write start timing on
a target surface different from the target surface based on the
corrected detection data.
3. The optical scanning apparatus according to claim 1, wherein the
processing unit is configured to determine a write start timing on
the target surface based on the corrected detection data.
4. The optical scanning apparatus according to claim 1, wherein the
processing unit is configured to correct the detection data of the
sync detecting sensor in accordance with the formula: (the
corrected detection data)=(a measured value T of a time needed for
one revolution of the rotary polygon mirror)/(an average Tave of
history data of the measured value).
5. The optical scanning apparatus according to claim 2, wherein the
processing unit is configured to determine a first write start
timing on a first target surface based on the detection data of the
sync detecting sensor, and determine a second write start timing on
a second target surface based on the corrected detection data, and
wherein a face of the polygon mirror which deflects the light beam
used to determine the first write start timing is the same as the
face of the polygon mirror which deflects the light beam used to
determine the second write start timing.
6. The optical scanning apparatus according to claim 3, wherein the
light source and the sync detecting sensor are mounted on a single
substrate.
7. An image forming device comprising: a photoconductive drum; and
the optical scanning apparatus according to claim 1, wherein the
optical scanning apparatus is arranged to optically scan the
photoconductive drum by the light beam.
8. An image forming device including a processing unit to control
the image forming device, the processing unit comprising: a first
sync signal generating part configured to detect scanning timing of
a first photoconductor and generate a first sync signal; a first
counter configured to receive the first sync signal and a first
pixel clock and measure a rotational period of a rotary polygon
mirror; a first pixel clock generating part configured to adjust
the frequency of the first pixel clock to make the rotational
period measured by the first counter equal to a predetermined
value; and a false sync signal generating part configured to
generate a false sync signal for measuring timing of scanning of a
second photoconductor, based on the first sync signal and the first
pixel clock.
9. The image forming device according to claim 8, wherein the first
pixel clock generating part outputs the first pixel clock and a
frequency correction value which is a difference between a
frequency of the first pixel clock and a predetermined frequency,
and wherein the processing unit further comprises: a second pixel
clock generating part configured to output a second pixel clock
that is in sync with the first sync signal, at a frequency
corrected by the frequency correction value; and a third pixel
clock generating part configured to output a third pixel clock that
is in sync with the false sync signal, at the frequency corrected
by the frequency correction value.
10. The image forming device according to claim 8, wherein the
first counter is configured to measure an input interval of the
first sync signal for each of the faces of the polygon mirror and
output an average of the intervals of the first sync signal
measured for the number of the faces of the polygon mirror.
11. The image forming device according to claim 9, wherein an
initial value of the frequency of the second pixel clock and an
initial value of the frequency of the third pixel clock are
predetermined individually.
12. The image forming device according to claim 11, wherein the
processing unit further comprises: a fourth pixel clock generating
part configured to generate a fourth pixel clock which is
oscillated in sync with the first sync signal at a frequency
obtained from a first initial frequency setting value corrected by
the frequency correction value, or oscillated in sync with the
false sync signal at a frequency obtained from a second initial
frequency setting value corrected by the frequency correction
value.
13. The image forming device according to claim 8, wherein the
processing unit further comprises: a second sync signal generating
part configured to detect a timing of scanning of a third
photoconductor by a third light beam deflected by the polygon
mirror and generate a second sync signal; and a second false sync
signal generating part configured to generate a second false sync
signal which measures timing of scanning of a fourth photoconductor
by a fourth light beam deflected by the polygon mirror, using the
first pixel clock.
14. The image forming device according to claim 13, further
comprising: a fifth pixel clock generating part configured to
output a fifth pixel clock that is in sync with the second sync
signal at a frequency corrected by the frequency correction value;
and a sixth pixel clock generating part configured to output a
sixth pixel clock that is in sync with the second false sync signal
at the frequency corrected by the frequency correction value.
15. The image forming device according to claim 14, wherein an
initial value of the frequency of the second pixel clock, an
initial value of the frequency of the third pixel clock, an initial
value of the frequency of the fifth pixel clock, and an initial
value of the frequency of the sixth pixel clock are predetermined
individually.
16. The image forming device according to claim 15, wherein the
processing unit further comprises: a seventh pixel clock generating
part configured generate a seventh pixel clock which is oscillated
in sync with the second sync signal at a frequency obtained from a
third initial frequency setting value corrected by the frequency
correction value, or oscillated in sync with the second false sync
signal at a frequency obtained from a fourth initial frequency
setting value corrected by the frequency correction value.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present disclosure relates to an optical scanning
apparatus which scans a target surface by light, and an image
forming device including the optical scanning apparatus provided
therein.
[0003] 2. Description of the Related Art
[0004] Image forming devices, such as digital multi-functional
peripherals or laser printers, are provided with an optical
scanning apparatus. In the optical scanning apparatus, a light beam
emitted from a light source is deflected by an optical deflector
having a rotary polygon mirror, so that a photoconductor drum
surface is scanned by the deflected light beam.
[0005] Generally, an image forming device including plural
photoconductor drums uses an optical scanning apparatus having two
scanning lenses arranged in two opposed positions that confront
opposite sides of the optical deflector respectively. In the
following, the optical scanning apparatus of this type will be
called an opposed scanning type optical scanning apparatus.
[0006] The optical scanning apparatus may include a sync detecting
sensor arranged at a predetermined position where a light beam
prior to a write start time is received, in order to equalize write
start positions for plural scanning lines in a main scanning
direction on the photoconductor drum surface. In the following, an
output signal of the sync detecting sensor will be called a sync
detection signal.
[0007] In the opposed scanning type optical scanning apparatus, a
single sync detecting sensor may be arranged at a position
confronting one of the photoconductor drums. In this case, a false
sync signal (or pseudo sync signal) for the other photoconductor
drums is generated based on the sync detection signal output from
the sync detecting sensor confronting the one of the photoconductor
drums.
[0008] Mirror surfaces of the rotary polygon mirror in the
above-mentioned optical deflector are formed by cutting. However,
at a result of a normal cutting process, the angle between two
adjacent ones of the mirror surfaces may not be uniform. In such a
case, when the photoconductor drum surface is scanned by the light
beam deflected on a mirror surface different from the mirror
surface from which the sync detection signal is obtained, if a
write start timing of an image is determined from a false sync
signal generated based on the sync detection signal, the write
start position of the image will deviate from the correct position.
If the precision of cutting is improved, the irregularity of the
angle may be reduced. However, the process cost will increase
conversely.
[0009] For example, Japanese Patent No. 4,393,133 discloses an
image forming device which is arranged to include a detection unit
arranged at a position corresponding to a first light emitting
device to detect a laser beam from the first light emitting device
scanned by a rotary polygon mirror. A signal generating unit
generates a horizontal sync signal for determining the timing to
form an electrostatic latent image in a main scanning direction on
an image support object with the laser beam from the first light
emitting device according to a result of the detection by the
detection unit. A measuring unit measures an interval of the
detection times that the laser beams of the first light emitting
device scanned by the faces of the polygon mirror are detected by
the detection unit sequentially. A timing determination unit
determines the timing to form the electrostatic latent image on the
image support object by a laser beam of a second light emitting
device scanned by a face of the polygon mirror different from the
face by which the laser beam of the first light emitting device is
scanned, based on the detection of the laser beam of the first
light emitting device by the detection unit when generating the
horizontal sync signal, and the interval measured by the measuring
part, without detecting the laser beam of the second light emitting
device to be scanned by the polygon mirror.
[0010] Japanese Laid-Open Patent Publication No. 2003-185952
discloses an optical scanning apparatus in which two or more of
plural scanning units share a polygonal deflector. The two or more
scanning units use the light beams deflected by different
deflection surfaces of the polygonal deflector. A single write
start position detecting unit is arranged to detect the light beams
from the different deflection surfaces of the polygonal deflector.
The write start timing for the scanning units to the target surface
is determined by using an output signal of this write start
position detecting part.
[0011] Moreover, Japanese Laid-Open Patent Publication No.
2004-102276, Japanese Laid-Open Patent Publication No. 2006-305780,
Japanese Patent No. 3,773,884, and Japanese Laid-Open Patent
Publication No. 2011-011504 disclose the related technology similar
to that of the above-mentioned related art documents.
[0012] However, in these years, the requirements for increasingly
high image quality of image forming devices are present, and there
has been a problem that the image forming device disclosed in
Japanese Patent No. 4,393,133 and the image forming device using
the optical scanning apparatus disclosed in Japanese Laid-Open
Patent Publication No. 2003-185952 have difficulty in satisfying
the requirements.
[0013] The inventors of the present application have examined the
image quality of an image forming device including an optical
scanning apparatus in which a false sync signal is generated to
determine a write start timing, and have discovered that the image
quality is affected by the rotation irregularity of the rotary
polygon mirror.
[0014] Moreover, as known conventionally, the error (the
face-by-face error) for the deflection reflective surfaces of the
polygon mirror is also a problem that affects the image
quality.
[0015] The main factor of the face-by-face error is the variations
in the distance from the rotation axis of the polygon mirror to the
deflection reflective surfaces of the optical deflector (or
eccentricity of the polygon mirror and profile irregularities of
the faces of the polygon mirror).
[0016] One of the causes of the irregularity of the write end
position is that the scanning irregularity is produced when two or
more light sources are used. The main factor of this scanning
irregularity is that there is a difference in the oscillation
wavelength of the light sources, and the scanning speed is varied
according to the chromatic aberration of the scanning optical
system.
SUMMARY OF THE INVENTION
[0017] Accordingly, in one aspect, the present disclosure provides
an optical scanning apparatus including: a light source; an optical
deflector having a rotary polygon mirror to deflect a light beam
from the light source; a scanning optical system arranged to focus
the light beam deflected by the optical deflector on a target
surface; a sync detecting sensor arranged to determine a write
start timing on the target surface; and a processing unit
configured to correct detection data of the sync detecting sensor
based on a measured value of a time needed for one revolution of
the rotary polygon mirror.
[0018] Other objects, features and advantages of the present
disclosure will become more apparent from the following detailed
description when read in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a diagram showing the composition of a color
printer of an embodiment of the present disclosure.
[0020] FIG. 2 is a diagram showing the composition of an optical
scanning apparatus shown in FIG. 1.
[0021] FIG. 3 is a diagram for explaining the faces of a polygon
mirror.
[0022] FIG. 4 is a timing chart for explaining a write start timing
in a photoconductor drum.
[0023] FIG. 5 is a timing chart for explaining a conventional
example 1 of a false sync signal.
[0024] FIG. 6 is a timing chart for explaining a conventional
example 2 of a false sync signal.
[0025] FIG. 7 is a timing chart for explaining a conventional
example 3 of a false sync signal.
[0026] FIG. 8 is a timing chart for explaining a method of
obtaining a time difference between the deflection reflective
surfaces in FIG. 7.
[0027] FIG. 9 is a diagram for explaining a time T1.
[0028] FIG. 10 is a diagram for explaining a time T2.
[0029] FIG. 11 is a diagram for explaining measurement data of time
T1 and time T2.
[0030] FIG. 12 is a diagram for explaining the computation result
of .DELTA.T2 and .DELTA.T2a.
[0031] FIG. 13A and FIG. 13B are diagrams for explaining a false
sync signal when taking the rotation irregularity into
consideration.
[0032] FIG. 14 is a flowchart for explaining a method of correcting
a coefficient k.
[0033] FIG. 15 is a diagram for explaining the method of correcting
the coefficient k.
[0034] FIG. 16 is a diagram for explaining an example of line
patterns.
[0035] FIG. 17 is a diagram for explaining an amount of deviation
.DELTA.L of the line patterns.
[0036] FIG. 18 is a diagram for explaining correction of the
rotation irregularity by a second method.
[0037] FIG. 19 is a diagram for explaining an optical scanning
apparatus in FIG. 1.
[0038] FIG. 20 is a diagram for explaining a case where a light
source and a sync detecting sensor are mounted on the same
substrate.
[0039] FIG. 21 is a diagram for explaining a case where a light
source and a sync detecting sensor are mounted on different
substrates.
[0040] FIG. 22A is a timing chart for explaining the case of FIG.
21.
[0041] FIG. 22B is a timing chart for explaining the case of FIG.
20.
[0042] FIG. 23 is a timing chart for explaining a time from a sync
detection instant to a write start instant when taking the rotation
irregularity into consideration.
[0043] FIG. 24 is a diagram showing the composition of the image
forming device of the first embodiment.
[0044] FIG. 25 is a block diagram showing the composition of a
pixel clock generating part-1.
[0045] FIG. 26 is a timing chart for explaining operation of the
pixel clock generating part-1.
[0046] FIG. 27 is a block diagram showing the composition of a
false signal generating part.
[0047] FIG. 28 is a diagram for explaining an error for each of the
faces of the polygon mirror.
[0048] FIG. 29 is a block diagram showing the composition of a
pixel clock generating part 2.
[0049] FIG. 30 is a diagram showing the composition of the image
forming device of the second embodiment.
[0050] FIG. 31 is a timing chart for explaining operation of the
false sync signal generating part in the second embodiment.
[0051] FIG. 32 is a diagram showing the composition of the image
forming device of the third embodiment.
[0052] FIG. 33 is a block diagram showing the composition of a
pixel clock generating part 4.
[0053] FIG. 34 is a diagram for explaining the relationship between
a pixel clock and image data.
[0054] FIG. 35 is a diagram showing the composition of the image
forming device of the fourth embodiment.
[0055] FIG. 36 is a timing chart for explaining operation of the
false sync signal generating part in the fourth embodiment.
[0056] FIG. 37 is a timing chart in which a first sync signal and a
second sync signal are separated from a sync signal.
[0057] FIG. 38 is a diagram showing the composition of the image
forming device of the fifth embodiment.
[0058] FIG. 39 is a diagram showing the composition of the image
forming device of the sixth embodiment.
[0059] FIG. 40 is a timing chart for explaining operation of the
false sync signal generating part in the sixth embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0060] A description will be given of embodiments of the present
disclosure with reference to the accompanying drawings.
[0061] FIG. 1 shows the outline composition of a color printer 2000
of an embodiment of the present disclosure. As shown in FIG. 1, the
color printer 2000 is a tandem-type multicolor printer which forms
a full color image by overlapping images of four colors (black,
cyan, magenta, yellow). The color printer 2000 includes two optical
scanning apparatuses (2010A, 2010B), four photoconductor drums
(2030a, 2030b, 2030c, 2030d), four cleaning units (2031a, 2031b,
2031c, 2031d), four charging units (2032a, 2032b, 2032c, 2032d),
four developing rollers (2033a, 2033b, 2033c, 2033d), a transfer
belt 2040, a transfer roller 2042, a fixing roller 2050, a feed
roller 2054, a delivery roller 2058, a sheet feed tray 2060, a
sheet output tray 2070, a communication control device 2080, and a
printer control device 2090 that controls the component parts of
the color printer.
[0062] The communication control device 2080 controls the
bidirectional communications between the color printer 2000 and a
host device (for example, a personal computer) through a
network.
[0063] The printer control device 2090 includes a CPU, a ROM which
stores programs executable by the CPU and various data used when
executing the programs, a RAM which provides a working area for the
CPU, and an AD (analog-to-digital) converter which converts analog
data into digital data. The printer control device 2090 controls
the component parts of the color printer 2000 according to the
instructions from the host device.
[0064] In the color printer 2000, the photoconductor drum 2030a,
the charging unit 2032a, the developing roller 2033a, and the
cleaning unit 2031a are used as a group and constitute a black
image formation station (which will be called "K station") that
forms an image of black. The photoconductor drum 2030b, the
charging unit 2032b, the developing roller 2033b, and the cleaning
unit 2031b are used as a group and constitute a cyan image
formation station (which will be called "C station") that forms an
image of cyan. The photoconductor drum 2030c, the charging unit
2032c, the developing roller 2033c, and the cleaning unit 2031c are
used as a group and constitute a magenta image formation station
(which will be called "M station") that forms an image of magenta.
The photoconductor drum 2030d, the charging unit 2032d, the
developing unit 2033d, and the cleaning unit 2031d are used as a
group and constitute a yellow image formation station (which will
be called "Y station") that forms an image of yellow.
[0065] On the surface of each of the photoconductor drums, a
photosensitive layer is formed. Namely, the surface of each
photoconductor drum is a target surface to be scanned, and a latent
image is formed on the target surface. Each of the photoconductor
drums is rotated around its horizontal rotation axis in a direction
indicated by the arrow in FIG. 1 by a non-illustrated rotating
mechanism.
[0066] Each of the charging units electrically charges the surface
of the corresponding one of the photoconductor drums in a uniform
manner respectively.
[0067] The optical scanning apparatus 2010A scans the charged
surface of each of the photoconductor drum 2030a and the
photoconductor drum 2030b by the light which is modulated for each
color based on the corresponding one of the image information of
black and the image information of cyan received from the printer
control device 2090 respectively. Thereby, the latent images
corresponding to the image information received are formed on the
surfaces of these photoconductor drums respectively. The latent
images formed are moved in the direction toward the corresponding
one of the developing rollers in accordance with the rotation of
the corresponding photoconductor drum.
[0068] The optical scanning apparatus 2010B scans the charged
surface of each of the photoconductor drum 2030c and the
photoconductor drum 2030d by the light which is modulated for each
color based on the corresponding one of the image information of
magenta and the image information of yellow received from the
printer control device 2090 respectively. Thereby, the latent
images corresponding to the image information received are formed
on the surfaces of these photoconductor drums respectively. The
latent images formed are moved in the direction toward the
corresponding one of the developing rollers in accordance with the
rotation of the corresponding photoconductor drum.
[0069] The details of each of the optical scanning apparatuses will
be described later.
[0070] In the following, a scanning area of each photoconductor
drum in which the image information is written will be called
"effective scanning area", "image formation area", or "effective
image area".
[0071] Toner from a corresponding toner cartridge (not shown) is
supplied to the surface of each of the developing rollers uniformly
by the rotation of the developing roller. When the toner on the
surface of each developing roller touches the surface of the
corresponding photoconductor drum, the toner adheres to only the
portion of the photoconductor drum surface which has been
irradiated with the light beam. Namely, each developing roller
serves to visualize the latent image on the surface of the
corresponding photoconductor drum into a toner image by applying
the toner thereto. This toner image is moved in the direction
toward the transfer belt 2040 in accordance with the rotation of
the photoconductor drum.
[0072] The respective toner images of yellow, magenta, cyan, and
black are sequentially transferred onto the transfer belt 2040 in
predetermined timing, so that these toner images are overlapped to
form a full color image.
[0073] A number of recording sheets is stored in the sheet feed
tray 2060. The feed roller 2054 is disposed in the vicinity of the
sheet feed tray 2060. The feed roller 2054 picks out a single
recording sheet from the recording sheets in the sheet feed tray
2060 at a time. This recording sheet is delivered to a nip between
the transfer belt 2040 and the transfer roller 2042 in
predetermined timing. Thereby, the color image on the transfer belt
2040 is transferred to the recording sheet. The recording sheet to
which the color image is transferred is delivered to the fixing
roller 2050.
[0074] In the fixing roller 2050, heat and pressure are applied to
the recording sheet so that the toner is fixed to the recording
sheet. The recording sheet to which the toner is fixed is delivered
to the sheet output tray 2070 through the delivery roller 2058. In
this manner, the recording sheets are sequentially accumulated on
the sheet output tray 2070.
[0075] Each of the cleaning units removes the remaining toner
(residual toner) from the surface of the corresponding
photoconductor drum. The surface of the photoconductor drum after
the remaining toner is removed is returned back to the position
which confronts the corresponding charging unit.
[0076] Next, the composition of the optical scanning apparatus
2010A will be explained.
[0077] As shown in FIG. 2, the optical scanning apparatus 2010A
includes two light sources (2200a, 2200b), two coupling lenses
(2201a, 2201b), two aperture plates (2203a, 2203b), two cylindrical
lenses (2204a, 2204b), a polygon mirror 2104A, two scanning lenses
(2105a, 2105b), two feedback mirrors (2106a, 2106b), a focusing
lens 2112A, a sync detecting sensor 2113A, and a non-illustrated
scanning control device A.
[0078] In the following, it is assumed that in an XYZ
three-dimensional rectangular coordinate system, the Y axis
direction is a direction parallel to the longitudinal direction (or
the rotational shaft direction) of each photoconductor drum, and
the Z axis direction is a direction parallel to the rotating shaft
of the polygon mirror 2104A.
[0079] Each light source includes a semiconductor laser and a drive
circuit which drives the semiconductor laser. The drive circuit of
each light source is controlled by the scanning control device
A.
[0080] In the following, a light beam emitted from the light source
2200a is called "light beam LBa", and a light beam emitted from the
light source 2200b is called "light beam LBb".
[0081] The coupling lens 2201a converts the light beam LBa emitted
from the light source 2200a into a generally parallel light beam.
The coupling lens 2201b converts the light beam LBb emitted from
the light source 2200b into a generally parallel light beam.
[0082] The aperture plate 2203a has an opening and adjusts the beam
diameter of the light beam LBa from the coupling lens 2201a. The
aperture plate 2203b has an opening and adjusts the beam diameter
of the light beam LBb from the coupling lens 2201b.
[0083] The cylindrical lens 2204a converges the light beam LBa
passed through the opening of the aperture plate 2203a with respect
to the Z axis direction near the deflection reflective surface of
the polygon mirror 2104A. The cylindrical lens 2204b converges the
light beam LBb having passed through the opening of the aperture
plate 2203b with respect to the Z axis direction near the
deflection reflective surface of the polygon mirror 2104A. The
optical system arranged between each light source and the polygon
mirror 2104A is also called the optical system in front of the
deflector.
[0084] The polygon mirror 2104A is a six-face mirror as a rotary
polygon mirror, and each mirror surface serves as a deflection
reflective surface. This rotary polygon mirror is uniformly rotated
around the rotating shaft thereof by a non-illustrated polygon
motor, and each face of the polygon mirror deflects the light from
each cylindrical lens at a constant angular velocity.
[0085] In the following, it is assumed that the rotary polygon
mirror is rotated clockwise. The polygon motor is controlled based
on an external clock signal so that the rotational frequency of the
rotary polygon mirror is set to 33300 rpm. Hence, the time for one
revolution of the rotary polygon mirror is about 1.8 ms.
[0086] The light beam LBa from the cylindrical lens 2204a enters
the deflection reflective surface of the polygon mirror 2104A
located on the -X side of the rotating shaft of the polygon mirror
2104A, and the light beam LBb from the cylindrical lens 2204b
enters the deflection reflective surface of the polygon mirror
2104A located on the +X side of the rotating shaft.
[0087] The scanning lens 2105a is located on the -X side of the
polygon mirror 2104A and arranged on the optical path of the light
beam LBa deflected by the polygon mirror 2104A.
[0088] The feedback mirror 2106a guides the light beam LBa from the
scanning lens 2105a to the photoconductor drum 2030a. That is, the
surface of the photoconductor drum 2030a is irradiated with the
light beam LBa, and a light spot is formed on the surface of the
photoconductor drum 2030a.
[0089] The scanning lens 2105b is located on the +X side of the
polygon mirror 2104A and arranged on the optical path of the light
beam LBb deflected by the polygon mirror 2104A.
[0090] The feedback mirror 2106b guides the light beam LBb from the
scanning lens 2105b to the photoconductor drum 2030b. That is, the
surface of the photoconductor drum 2030b is irradiated with the
light beam LBb, and a light spot is formed on the surface of the
photoconductor drum 2030b.
[0091] The light spot on the surface of each photoconductor drum is
moved in the longitudinal direction of the photoconductor drum in
accordance with the rotation of the polygon mirror 2104A. The
moving direction of the light spot at this time is called "main
scanning direction", and the direction of rotation of the
photoconductor drum is called "sub-scanning direction".
[0092] The sync detecting sensor 2113A is arranged in a position
which receives, through the focusing lens 2112A, a light beam
directed to the outside of the effective scanning area of the
photoconductor drum 2030b. The sync detecting sensor 2113A outputs
a sync detection signal to the scanning control device A.
[0093] The sync detecting sensor 2113A is configured so that the
sync detection signal is set to "low level" when the amount of
received light is smaller than a predetermined value, and set to
"high level" when the amount of received light is larger than the
predetermined value. That is, when the sync detecting sensor 2113A
receives a light beam, the sync detection signal changes from "high
level" to "low level".
[0094] The scanning control device A determines a write start
timing for the surface of the photoconductor drum 2030b based on
the output of the sync detecting sensor 2113A (the sync detection
signal).
[0095] FIG. 3 shows an example of the faces of the polygon mirror
2104A. As shown in FIG. 3, the six deflection reflective surfaces
of the polygon mirror 2104A are called "face-1", "face-2",
"face-3", "face-4", "face-5", and "face-6" counterclockwise.
[0096] For example, when the photoconductor drum 2030b is first
scanned by the light reflected on the face-1, the photoconductor
drum 2030a is next scanned by the light reflected on the face-3.
Then, the photoconductor drum 2030a is scanned by the light
reflected on the face-2, and the photoconductor drum 2030b is
scanned by the light reflected on the face-4. Further, the
photoconductor drum 2030a is scanned by the light reflected on the
face-3, and the photoconductor drum 2030b is scanned by the light
reflected on the face-5.
[0097] FIG. 4 is a timing chart for explaining a write start timing
in the photoconductor drum 2030b. As shown in FIG. 4, if a rise of
the output of the sync detecting sensor 2113A is detected, the
scanning control device A starts the writing to the photoconductor
drum 2030b after progress of a time Tk from the instant of the
detection. This time Tk is an interval from the timing of the rise
of the sync detection signal to the write start timing, which is
predetermined for each image forming device and stored in the
memory of the scanning control device A.
[0098] In this embodiment, a sync detecting sensor corresponding to
the photoconductor drum 2030a is not arranged, and a sync detection
signal regarding the photoconductor drum 2030a cannot be received.
In this case, a method of determining the write start timing for
the photoconductor drum 2030a is shown in FIG. 5. FIG. 5 shows a
method of generating a false sync signal in sync with the output of
the sync detecting sensor 2113A (conventional example 1). As shown
in FIG. 5, this false sync signal is a signal which changes from
"low level" to "high level" when a time Tr passes from the timing
of a rise of the output of the sync detecting sensor 2113A. The
time Tr is a time needed for 1/6 of one revolution of the polygon
mirror, which is predetermined for each image forming device.
[0099] Conventionally, as shown in FIG. 5, the writing to the
photoconductor drum 2030a is started after progress of the time Tk
from the timing of a rise of the false sync signal. However, a
manufacturing error may exist in the polygon mirror, and there is a
problem that, if the polygon mirror has an error in the deflection
reflective surfaces, the write start positions differ.
[0100] A first method for eliminating the problem is, as shown in
FIG. 6 (conventional example 2), to determine a write start timing
when scanning the photoconductor drum 2030a using the light
reflected on the same deflection reflective surface based on the
output of the sync detecting sensor 2113A. In FIG. 6, a time Tr'
indicates an interval corresponding to an integral multiple of the
time Tr above.
[0101] A second method for eliminating the problem is, as shown in
FIG. 7 (conventional example 3), that time differences between the
deflection reflective surfaces (Te13, Te24, Te35, Te46, Te51, Te62)
are computed beforehand, the time Tr is corrected, and the write
start timing for the photoconductor drum 2030a is determined by
taking the time differences into consideration. In FIG. 7, Te13
denotes a time difference between the face-1 and the face-3, Te24
denotes a time difference between the face-2 and the face-4, and
Te35 denotes a time difference between the face-3 and the face-5.
Te46 denotes a time difference between the face-4 and the face-6,
Te51 denotes a time difference between the face-5 and the face-1,
and Te62 denotes a time difference between the face-6 and the
face-2.
[0102] In order to perform the method of FIG. 7, it is necessary to
compute the time differences between the deflection reflective
surfaces beforehand. First, in this method, as shown in FIG. 8, the
following times are measured based on the output of the sync
detecting sensor 2113A: a time T12 for the face-1 (as the
deflection reflective surface reflecting the light beam) to be
moved to the face-2; a time T23 for the face-2 (as the deflection
reflective surface reflecting the light beam) to be moved to the
face-3; a time T34 for the face-3 (as the deflection reflective
surface reflecting the light beam) to be moved to the face-4; a
time T45 for the face-4 (as the deflection reflective surface
reflecting the light beam) to be moved to the face-5; a time T56
for the face-5 (as the deflection reflective surface reflecting the
light beam) to be moved to the face-6; and a time T61 for the
face-6 (as the deflection reflective surface reflecting the light
beam) to be moved to the face-1.
[0103] Next, a difference of the time T12 and the time Tr is
computed and the time difference Te12 between the face-1 and the
face-2 is set to the computed difference. A difference of the time
T23 and the time Tr is computed and the time difference Te23
between the face-2 and the face-3 is set to the computed
difference. A difference of the time T34 and the time Tr is
computed and the time difference Te34 between the face-3 and the
face-4 is set to the computed difference. A difference of the time
T45 and the time Tr is computed and the time difference Te45
between the face-4 and the face-5 is set to the computed
difference. A difference of the time T56 and the time Tr is
computed and the time difference Te56 between face-5 and face-6 is
set to the computed difference. A difference of the time T61 and
the time Tr is computed and the time difference Te61 between the
face-6 and the face-1 is set to the computed difference.
[0104] A time difference Te13 is determined by a sum of the time
difference Te12 and the time difference Te23. A time difference
Te24 is determined by a sum of the time difference Te23 and the
time difference Te34. A time difference Te35 is determined by a sum
of the time difference Te34 and the time difference Te45. A time
difference Te46 is determined by a sum of the time difference Te45
and the time difference Te56. A time difference Te51 is determined
by a sum of the time difference Te56 and the time difference Te61.
A time difference Te62 is determined by a sum of the time
difference Te61 and the time difference Te12.
[0105] In any of the above-mentioned methods, the elapsed time from
the reception of the light beam at the sync detecting sensor 2113A
to the write start timing for the photoconductor drum 2030a is
longer than the elapsed time in the case where the sync detecting
sensor corresponding to the photoconductor drum 2030a is
disposed.
[0106] The inventors of the present application have examined the
image quality of an image forming device including an optical
scanning apparatus in which a false sync signal is generated to
determine the write start timing, and have discovered that the
image quality is affected by the rotation irregularity of the
rotary polygon mirror. Moreover, the inventors have discovered that
the larger the elapsed time from the reception of the light beam at
the sensor to the write start timing, the larger the influence by
the rotation irregularity of the rotary polygon mirror.
[0107] As an example, as shown in FIG. 9, a time from the reception
of the light beam reflected on one deflection reflective surface
(the face-1 in FIG. 9) at the sync detecting sensor 2113A to the
reception of the light beam reflected again on the same deflection
reflective surface is set to T1. Namely, this time T1 is a measured
value of a time for one revolution of the rotary polygon
mirror.
[0108] As an example, as shown in FIG. 10, it is assumed that a
virtual sync detecting sensor corresponding to the photoconductor
drum 2030a (which sensor is indicated as a false sync detecting
sensor in FIG. 10) is arranged in the position which outputs the
false sync signal as shown in FIG. 5. In this case, a time from the
reception of the light beam reflected on one deflection reflective
surface (the face-1 in FIG. 10) at the sync detecting sensor 2113A
to the reception of the light beam reflected on the same deflection
reflective surface at the false sync detecting sensor is set to T2
(<T1).
[0109] Conventionally, it was considered that both the time T1 and
the time T2 have a constant value. The inventors have measured the
time T1 and the time T2 experimentally. FIG. 11 shows the
measurement data (1000 measurement data pieces) of T1 until the
cumulative rotation time of the rotary polygon mirror reaches 300
ms, and the measurement data (1000 measurement data pieces) of T2
measured in sync with the measurement of T1.
[0110] As is apparent from FIG. 11, T1 and T2 were not constant and
both were varied. It can be understood that variations in T1 and T2
are due to the rotation irregularity of the rotary polygon
mirror.
[0111] In this example, the average of the 1000 measurement data
pieces of T1 was 1801.804 .mu.s and the average of the 1000
measurement data pieces of T2 was 1263.083 .mu.s. Even if the
cumulative rotation time is extended beyond 300 ms, there was no
significant difference in the values of these averages.
[0112] In the following, "T1ave" denotes the average of plural
measurement data pieces of T1, and "T2ave" denotes the average of
plural measurement data pieces of T2. A difference .DELTA.T1 of T1
and T1ave, and a difference .DELTA.T2 of T2 and T2ave are
represented by the following formulas:
.DELTA.T1=T1-T1ave (1)
.DELTA.T2=T2-T2ave (2)
[0113] In addition, a difference .DELTA.T2a of T2 and T2ave when
the T2ave is corrected by considering the rotation irregularity of
the rotary polygon mirror is represented by the following
formula.
.DELTA.T2a=T2-T2ave(T1-/T1ave) (3)
[0114] FIG. 12 shows the computation results of .DELTA.T2 and
.DELTA.T2a described above. As shown in FIG. 12, the variations of
.+-.0.015 .mu.s (=.+-.15 ns) were present for .DELTA.T2, and they
were reduced to the variations of .+-.0.004 .mu.s (=.+-.4 ns) which
were present for .DELTA.T2a.
[0115] According to this experiment, it can be understood that 73%
(=1-4/15) of the variations in T2 were due to the rotation
irregularity. It was confirmed that the variations in T2 are
greatly dependent on the variations in T1 (rotation
irregularity).
[0116] If T2 is set up to meet the condition .DELTA.T2a=0 (i.e.,
the following equation (4) is given), the influence of the rotation
irregularity in T2 can be reduced.
T2-T2ave(T1/T1ave)=0 (4)
[0117] If the above equation (4) is modified, the following
equation (5) is obtained.
T2=(T2ave/T1ave)T1 (5)
[0118] According to the experiments, T1ave=1801.804 .mu.s and
T2ave=1263.083 .mu.s, the following formula (6) is obtained from
the above equation (5):
T2=0.7010.times.T1 (6)
[0119] This shows that, if the false sync signal is changed from
low level to high level at the timing of 0.70101 revolutions of the
rotary polygon mirror after the reception of the light beam
reflected on one deflection reflective surface (e.g., the face-1 in
FIG. 10) by the sync detecting sensor 2113A, the influence of the
rotation irregularity can be prevented (see FIG. 13A and FIG.
13B).
[0120] However, in the image forming device, no sync detecting
sensor corresponding to the photoconductor drum 2030a is arranged,
and the measurement data of T2ave cannot be obtained.
[0121] To eliminate the problem, the above equation (5) is
rewritten into the following equation (7) using a coefficient
k.
T2=k.times.T1 (7)
[0122] FIG. 14 is a flowchart for explaining a method of
determining a coefficient k in the image forming device.
[0123] As shown in FIG. 14, at step S401, a default value of the
coefficient k indicating a time for one revolution of the rotary
polygon mirror from a time the sync detecting sensor 2113A receives
the light beam reflected on one deflection reflective surface (for
example, the face-1) to a time the above-described false sync
detecting sensor receives the light beam reflected on the
deflection reflective surface is computed.
[0124] At step S403, as shown in FIG. 15, a false sync signal is
generated using the default value of the coefficient k, and the
line patterns (see FIG. 16) which are the same as those used in the
known color matching correction process (see Japanese Laid-Open
Patent Publication No. 2011-197134) are formed. In FIG. 15,
T1.sub.1-T1.sub.4 denote the measurement values of T1 obtained at
the preceding cycle.
[0125] At step S405, a difference .DELTA.L (see FIG. 17) in the
position in the main scanning direction between the line pattern of
black and the line pattern of cyan is measured using the sensors
(not shown) used in the above-mentioned known color matching
correction process.
[0126] At step S407, the average of the above-mentioned .DELTA.L is
converted into an amount of rotation of the polygon mirror.
[0127] At step S409, the coefficient k is corrected based on the
amounts of rotation so as to set the average of the above-mentioned
.DELTA.L to 0. The corrected coefficient k is used as the
coefficient k of the above equation (7) for the image forming
device.
[0128] Upon power-up, the printer control device 2090 performs an
image process control procedure if any of the following events take
place: (1) the stop time of the photoconductor drum is over 6
hours; (2) the inside temperature of the device has changed by 10
degrees C. or more; (3) the relative humidity of the device has
changed by 50% or more; (4) the number of printed sheets has
reached a predetermined number in the printing job; (5) the number
of times of rotation of the developing roller has reached a
predetermined number; and (6) the total running distance of the
transfer belt has reached a predetermined distance.
[0129] Hence, the printer control device 2090 may be configured to
perform the above method of determining the coefficient k during
execution of the image process control procedure. In this case, the
corrected coefficient k obtained at the preceding cycle may be
used, instead of the default value. Thereby, the false sync signal
can be generated with a higher level of accuracy.
[0130] When the above-mentioned second method is used, the false
sync signal can be generated with a higher level of accuracy by
taking the rotation irregularity into consideration.
[0131] As shown in FIG. 18, Tr.times.(T1/T1ave) is used instead of
Tr. In FIG. 18, T1.sub.1-T1.sub.8 denote the measurement values of
T1 obtained at the preceding cycle.
[0132] It is expected that, due to a detection error of the sync
detecting sensor 2113A or the like, an unusual value is included in
the measured values of T1. When the measurement values of T1
obtained at the preceding cycle do not fall within a predetermined
range, the measurement values of T1 obtained before the preceding
cycle may be used. Alternatively, plural sets of the measurement
values of T1 for plural cycles may be stored as history
information, and the average of such values may be used.
[0133] Next, the composition of the optical scanning apparatus
2010B will be described. As shown in FIG. 19, this optical scanning
apparatus 2010B includes two light sources (2200c, 2200d), two
coupling lenses (2201c, 2201d), two aperture plates (2203c, 2203d),
two cylindrical lenses (2204c, 2204d), a polygon mirror 2104B, two
scanning lenses (2105c, 2105d), two feedback mirrors (2106c,
2106d), a focusing lens 2112B, a sync detecting sensor 2113B, and a
non-illustrated scanning control device B.
[0134] Each light source includes a semiconductor laser and a drive
circuit which drives the semiconductor laser. Each drive circuit is
controlled by the scanning control device B.
[0135] In the following, a light beam emitted from the light source
2200c is called "light beam LBc", and a light beam emitted from the
light source 2200d is called "light beam LBd".
[0136] The coupling lens 2201c converts the light beam LBc emitted
from the light source 2200c into a generally parallel light beam.
The coupling lens 2201d converts the light beam LBd emitted from
the light source 2200d into a generally parallel light beam.
[0137] The aperture plate 2203c has an opening and adjusts the beam
diameter of the light beam LBc from the coupling lens 2201c. The
aperture plate 2203d has an opening and adjusts the beam diameter
of the light beam LBd from the coupling lens 2201d.
[0138] The cylindrical lens 2204c converges the light beam LBc
having passed through the opening of aperture plate 2203c with
respect to the Z axis direction near the deflection reflective
surface of the polygon mirror 2104B. The cylindrical lens 2204d
converges the light beam LBd passed through the opening of the
aperture plate 2203d with respect to the Z axis direction near the
deflection reflective surface of the polygon mirror 2104B. The
optical system arranged between each light source and the polygon
mirror 2104B is also called the optical system in front of the
deflector.
[0139] The polygon mirror 2104B is a six-face mirror as a rotary
polygon mirror, and each mirror surface serves as a deflection
reflective surface. This polygon mirror is uniformly rotated around
the center of the rotating shaft of the polygon mirror 2104B
parallel to the Z axis direction, and each face of the polygon
mirror deflects the light from each cylindrical lens at a constant
angular velocity. In the following, it is assumed that the polygon
mirror is rotated clockwise.
[0140] The light beam LBc from the cylindrical lens 2204c enters
the deflection reflective surface located on the -X side of the
rotating shaft of the polygon mirror 2104B, and the light beam LBd
from the cylindrical lens 2204d enters the deflection reflective
surface located on the +X side of the rotating shaft.
[0141] The scanning lens 2105c is located on the -X side of the
polygon mirror 2104B, and arranged on the optical path of the light
beam LBc deflected by the polygon mirror 2104B.
[0142] The feedback mirror 2106c guides the light beam LBc from the
scanning lens 2105c to the photoconductor drum 2030c by reflection.
That is, the surface of the photoconductor drum 2030c is irradiated
with the light beam LBc, and a light spot is formed on the
photoconductor drum 2030c surface.
[0143] The scanning lens 2105d is located on the +X side of the
polygon mirror 2104B, and arranged on the optical path of the light
beam LBd deflected by the polygon mirror 2104B.
[0144] The feedback mirror 2106d guides the light beam LBd from the
scanning lens 2105d to the photoconductor drum 2030d by reflection.
That is, the surface of the photoconductor drum 2030d is irradiated
with the light beam LBd and a light spot is formed on the
photoconductor drum 2030d surface.
[0145] The light spot on the surface of each photoconductor drum is
moved in the longitudinal direction of the photoconductor drum in
accordance with the rotation of the polygon mirror 2104B. The
moving direction of the light spot at this time is called main
scanning direction, and the direction of rotation of the
photoconductor drum is called sub-scanning direction.
[0146] The sync detecting sensor 2113B is arranged in a position
which receives, through the focusing lens 2112B, a light beam
directed to the outside of the effective scanning area of the
photoconductor drum 2030d. The sync detecting sensor 2113B outputs
a sync detection signal to the scanning control device B.
[0147] The scanning control device B determines a write start
timing for the surface of the photoconductor drum 2030d based on
the sync detection signal output from the sync detecting sensor
2113B.
[0148] Similar to the scanning control device A, the scanning
control device B generates a false sync signal based on a measured
value of a time for one revolution of the rotary polygon mirror of
the sync detection signal output from the sync detecting sensor
2113B, and determines the write start timing for the surface of the
photoconductor drum 2030d.
[0149] As described above, according to the foregoing embodiment,
the optical scanning apparatus includes two light sources, two
coupling lenses, two aperture plates, two cylindrical lenses, a
polygon mirror, two scanning lenses, two feedback mirrors, a
focusing lens, a sync detecting sensor, and a scanning control
device. The scanning control device determines the write start
timing for the surface of the photoconductor drum corresponding to
the sync detecting sensor based on the output of the sync detecting
sensor. The scanning control device determines the write start
timing for the surface of the photoconductor drum with which no
sync detecting sensor is arranged, based on the output of the sync
detecting sensor and the measured value of the time for one
revolution of the rotary polygon mirror.
[0150] In this case, by taking into consideration the rotation
irregularity of the polygon mirror, the write start timing for the
surface of the photoconductor drum with which no sync detecting
sensor is arranged can be determined. It is possible to determine
the write start timing for the photoconductor drum with which no
sync detecting sensor is arranged with a high level of accuracy. As
a result, the variations of the write start position on the target
surface can be reduced.
[0151] The color printer 2000 is provided with the optical scanning
apparatus 2010A and the optical scanning apparatus 2010B. As a
result, the color printer 2000 can increase the image quality.
[0152] For the purpose of the cost reduction or miniaturization, a
sync detecting sensor may be mounted on a substrate on which the
light source is mounted (see FIG. 20). In this case, the time from
the reception of a light beam by the sync detecting sensor to the
write start timing is larger than that in the case where a sync
detecting sensor is mounted on another substrate different from the
substrate on which the light source is mounted (see FIG. 21), as
shown in FIG. 22A and FIG. 22B. The timing chart of FIG. 22A
illustrates the case of FIG. 21, and the timing chart of FIG. 22B
illustrates the case of FIG. 20.
[0153] The case in which the sync detecting sensor is mounted on
the substrate on which the light source is mounted is susceptible
to the influence of rotation irregularity, and there is a
possibility that the write start position is shifted. In this case,
the time from the reception of the light beam at the sync detecting
sensor to the write start timing is corrected by considering the
rotation irregularity as in the above-mentioned embodiment, and the
variations of the write start position can be reduced.
[0154] Specifically, as shown in FIG. 23, "t.times.(T1/T1ave)" is
used as the time from the reception of the light beam at the sync
detecting sensor to the write start timing, instead of the time "t"
computed according to the related art. In FIG. 23,
T1.sub.1-T1.sub.8 denote the measurement values of T1 obtained at
the preceding cycle.
[0155] Alternatively, a surface emission-type laser array having
plural emitting parts may be used as the semiconductor laser in
each light source in the above-mentioned embodiment.
[0156] Alternatively, the optical scanning apparatus 2010A and the
optical scanning apparatus 2010B in the above-mentioned embodiment
may be unified.
[0157] In the above-mentioned embodiment, the color printer 2000
including the four photoconductor drums has been explained as an
example of the image forming device of the present disclosure.
However, the present disclosure is not limited to this embodiment.
For example, the image forming device of the present disclosure is
applicable to a printer having two photoconductor drums or a
printer having five photoconductor drums.
[0158] In the above-mentioned embodiment, the case where the
optical scanning apparatus is used in the color printer has been
explained as an example of the present disclosure. However, the
present disclosure is not limited to this embodiment. For example,
the present disclosure is applicable to other image forming
devices, such as a copying machine, a facsimile machine, or a
multi-function peripheral.
[0159] Next, a first embodiment of the present disclosure will be
described. FIG. 24 is a diagram showing the composition of the
image forming device of the first embodiment of the present
disclosure.
[0160] As shown in FIG. 24, this image forming device includes two
light sources (2200a, 2200b), two coupling lenses (2201a, 2201b),
two aperture plates (2203a, 2203b), two cylindrical lenses (2204a,
2204b), a polygon mirror (2104A), two scanning lenses (2205a,
2205b), two feedback mirrors (2206a, 2206b), two photoconductors
2208a and 2208b, a focusing lens 2112A, a sync detecting sensor
2113A, and a pixel clock generating device 120.
[0161] The pixel clock generating device 120 includes a pixel clock
generating part-1 (111), a false sync signal generating part 113, a
pixel clock generating part-2 (112), a pixel clock generating
part-3 (114), a first modulation data generating part 115, a second
modulation data generating part 118, a first laser driver 116, and
a second laser driver 119.
[0162] A laser incident light beam 2207a from the light source
2200a enters the polygon mirror 2104A, and in sync with the
rotation of the polygon mirror 2104A, passes through the scanning
lens 2205a, so that the surface of the photoconductor 2208a is
scanned by the light beam. On the other hand, a laser incident
light beam 2207b from the light source 2200b enters the polygon
mirror 2104A, and in sync with the rotation of the polygon mirror
2104A, passes through the scanning lens 2205b, so that the surface
of the photoconductor 2208b is scanned by the light beam. Thereby,
an electrostatic latent image according to the output of the light
source 2200a is formed on the surface of photoconductor 2208a and
an electrostatic latent image according to the output of the light
source 2200b is formed on the surface of photoconductor 2208b.
[0163] The sync detecting sensor 2113A is disposed at an end
portion of the photoconductor 2208a. The laser beam reflected by
the polygon mirror 2104A enters the sync detecting sensor 2113A
before the scanning of a scanning line by the laser beam is
performed on the surface of the photoconductor 2208a. The sync
detecting sensor 2113A detects the timing of a start of the
scanning. The timing of the start of the scanning is detected by
the sync detecting sensor 2113A is supplied to the pixel clock
generating part-1 (111) and the false sync signal generating part
113 of the pixel clock generating device 120 as a first sync signal
that is present periodically in sync with the scanning of the
photoconductor.
[0164] The pixel clock generating part-1 (111) generates a first
pixel clock and a frequency correction value based on the first
sync signal. The false sync signal generating part 113 generates a
false sync signal based on the first sync signal and the first
pixel clock. The pixel clock generating part-2 (112) corrects the
initial frequency setting value by the frequency correction value
and generates a second pixel clock in sync with the first sync
signal. The pixel clock generating part-3 (114) corrects the
initial frequency setting value by the frequency correction value
and generates a third pixel clock in sync with the false sync
signal.
[0165] Based on the first image data, the first modulation data
generating part 115 outputs first modulation data to the first
laser driver 116, the first modulation data being synchronized with
the second pixel clock. The first laser driver 116 drives the light
source 2200a according to the first modulation data, and the light
source 2200a emits a laser beam. Based on the second image data,
the second modulation data generating part 118 outputs second
modulation data to the second laser driver 119, the second
modulation data being synchronized with the third pixel clock. The
second laser driver 119 drives the light source 2200b according to
the second modulation data, and the light source 2200b emits a
laser beam.
[0166] FIG. 25 is a block diagram showing the composition of the
pixel clock generating part-1 (111).
[0167] As shown in FIG. 25, the pixel clock generating part-1 (111)
includes a first counter 201, a moving-average computing unit 203,
a filter 204, a divider 205, a delay unit 206, a register 207, a
digital clock oscillator 208, a comparator 209, and an adder
210.
[0168] The first counter 201 outputs a counted value of the first
pixel clock indicating the interval of the first sync signal
corresponding to the interval in which one main scanning line for
the polygon mirror's one face amount is scanned. The comparator 209
compares the counted value with a reference number Nref of the
polygon mirror's one face amount, and supplies a difference of the
counted value and the reference number Nref to the moving-average
computing unit 203.
[0169] Assuming that E.sub.R denotes a ratio of effective scanning
period, .nu. denotes a photoconductor linear velocity, L denotes an
effective write width, .rho..sub.m denotes a picture element
density of main scanning direction, .rho..sub.s denotes a picture
element density of sub-scanning direction, and M denotes the number
of writing beams, the reference number Nref for the polygon
mirror's one face amount is represented by the following
formula:
Nref = vL E R .rho. m .rho. s 25.4 2 M ##EQU00001##
[0170] For example, when the number of the faces of the polygon
mirror is four, the moving-average computing unit 203 computes the
moving average of the difference values for the polygon mirror's
four face amount. The computed value of the moving average of the
difference values for the polygon mirror's four face amount is
smoothed by the filter 204, and divided by Nref at the divider 205,
so that it is converted into the error .DELTA.f now per period of
one pixel. The delay unit 206 adds the error .DELTA.f now per
period of one pixel to the frequency correction value .DELTA.f as a
control value. The adder 210 outputs the sum value of the frequency
correction value .DELTA.f and the initial frequency fclk_i stored
in the register 207. Then, the first pixel clock is generated by
the digital clock oscillator 208. By this feedback control, the
error per period of one pixel is made to fall within a
predetermined range. When the error is within the predetermined
range, the frequency correction value .DELTA.f at that time is
supplied to the pixel clock generating part-2 (112) and the pixel
clock generating part-3 (114).
[0171] The value of Nref and the initial frequency fclk_i of the
register 207 may be determined by the ratio E.sub.R of effective
scanning period, the photoconductor linear velocity .nu., the
effective write width L, the picture element density .rho..sub.m of
main scanning direction, the picture element density .rho..sub.s of
sub-scanning direction, and the writing beam number M of the image
forming device concerned.
[0172] FIG. 26 is a timing chart for explaining operation of the
pixel clock generating part-1 (111).
[0173] In the example of FIG. 26, the interval Tspsp of the first
sync signal for the polygon mirror's one face received from the PD
110 is varied as the rotational speed of the polygon mirror 100 and
an error arises. If fclk_w denotes the frequency of the first
control clock as a control value, the period of the first pixel
clock is represented by 1/fclk_w. The frequency fclk_w is
controlled to make the counted value of the first pixel clock equal
to the reference number Nref. At this time, the following
relationship is met.
Tspsp=Nref/fclk.sub.--w
[0174] On the other hand, if Tspsp_target denotes the target period
determined for each model of the image forming device and fclk_i
denotes the initial frequency, the following relationship is
met.
Tspsp_target=Nref/fclk.sub.--i
[0175] If .DELTA.f denotes the error of the frequency, the
following condition is met.
1/.DELTA.f=1/fclk.sub.--w-1/fclk.sub.--i
[0176] The time error .DELTA.t by the error of the scanning speed
is
.DELTA. t = Tspsp - Tspsp_target = Nref ( 1 - / fclk_w - 1 / fclk_i
) = Nref / .DELTA. f . ##EQU00002##
Namely, it is possible to correct the frequency of the first pixel
clock according to the rotational error of the polygon mirror by
comparing the rotational error for one revolution of the polygon
mirror with the reference number Nref (when no error between the
faces of the polygon mirror exists) multiplied by the number of
faces of the polygon mirror.
[0177] When scanning the photoconductor using a common polygon
mirror, a similar frequency error .DELTA.f arises in the second
pixel clock and the third pixel clock. Therefore, it is possible to
correct the frequency of each pixel clock according to the
rotational error for one revolution of the polygon mirror by
applying the frequency error .DELTA.f the second pixel clock and
the third pixel clock as a correction value.
[0178] FIG. 27 is a block diagram showing the composition of the
false signal generating part 113. As shown in FIG. 27, the false
sync signal generating part 113 includes a splitter 401, a second
counter 402, a comparator 403, and an OR circuit 404.
[0179] The first sync signal from the PD 110 is distributed to the
splitter 401 for the faces of the polygon mirror. For example, the
first sync signal of the 0th face of the polygon mirror is
distributed to the splitter 401-0, and the second counter 402-0
counts the number of the first pixel clocks (or the pixel number)
starting from the first sync signal of the 0th face. Similarly, the
first sync signal of the first face of the polygon mirror is
distributed to the splitter 401-1, and the second counter 402-1
counts the number of the first pixel clocks (or the pixel number)
starting from the first sync signal of the first face. The first
sync signal of the second face of the polygon mirror is distributed
to the splitter 401-2, and the second counter 402-2 counts the
number of the first pixel clocks (or the pixel number) starting
from the first sync signal of the second face. The counted value
after the counting is carried out is compared with a predetermined
false sync count number Nref_ps by the comparator 403. The output
of the comparator 403 is asserted when the counted value=the false
sync count number Nref_ps. The OR circuit 404 takes the OR of the
asserted outputs of the respective comparators and outputs a false
sync signal indicating the OR result.
[0180] In the example of FIG. 27, the polygon mirror having the
four faces, the splitter (the splitters 0-2), the counter (the
counters 0-2), and the comparator (the comparators 0-2) are
illustrated. The number of the elements necessary for each of the
splitter 401, the second counter 402, and the comparator 402 is
smaller than the number of faces of the polygon mirror. The number
and combination of the elements, such as the counter, may be
appropriately selected by the number of faces of the polygon mirror
used.
[0181] Next, an error of the radius of the inscribed circle of a
polygon mirror which may cause a deviation of a write start
position on a target surface to be scanned will be described. FIG.
28 is a diagram for explaining an error for each of the faces of
the polygon mirror.
[0182] In FIG. 28, "a" denotes a distance from the center of a
first polygon mirror to the face of the first polygon mirror, and
"b" denotes a distance from the center of a second polygon mirror
to the face of the second polygon mirror. If the incident light
enters at the same position but the center-to-face distances of the
polygon mirrors differ, the light beam is reflected at different
positions on the polygon mirror faces, and the write positions on
the photoconductor surfaces differ. Hence, if a false sync signal
is generated with respect to the polygon mirror face which is the
same as the face of the polygon mirror where the counting is
started, the above-described error does not arise. Therefore, the
false sync count number Nref_ps must be determined such that a
false sync signal is generated with respect to the polygon mirror
face which is the same as the face of the polygon mirror where the
counting is started. The false sync count number Nref_ps is stored
in a non-illustrated register. In addition, there may be a case in
which a mirror angle error of the polygon mirror is taken into
consideration.
[0183] The write start positions of the photoconductor drum 103 and
the photoconductor drum 104 may be shifted due to the errors
specific to the scanning optical system, such as mounting positions
of the scanning lenses 101 and 102, and manufacture errors of lens
curved surfaces. As disclosed in Japanese Laid-Open Patent
Publication No. 2004-102276, there is known a method of detecting a
write start position. In this method, predetermined positioning
marks are used as reference marks when combining the images of the
photoconductor drums 103 and 104, and detected by positioning
sensors, so that the amount of deviation is computed based on the
detection results of the sensors.
[0184] The false sync count number Nref_ps is set up based on the
computation result of the amount of deviation so as to correct the
error for each scanning optical system.
[0185] FIG. 29 is a block diagram showing the composition of the
pixel clock generating part-2 (112). The composition of the pixel
clock generating part-2 (112) will be described. In this respect,
the pixel clock generating part-3 (114) has the same composition as
the pixel clock generating part-2 (112), and a description thereof
will be omitted.
[0186] As shown in FIG. 29, the pixel clock generating part-2 (112)
includes an initial frequency setting unit 602, an adder 603, and a
digital control oscillator (DCO) 604. The initial frequency setting
unit 602 stores an initial value of the frequency of the pixel
clock 2. The adder 603 adds the frequency correction value
generated by the first pixel clock generating part to the initial
frequency, so that the rotational error of the polygon mirror is
corrected. The adder 603 outputs the resulting frequency setting
value (in which the rotational error of the polygon mirror is
corrected) to the digital control oscillator 604. Taking the phase
synchronization to the first sync signal, the digital control
oscillator (DCO) 604 generates the second pixel clock based on the
frequency set up by the adder.
[0187] In the pixel clock generating part-2 (112), an initial value
different from that of the first pixel clock generating part can be
set up, and it is possible to perform the setting of the initial
value so as to correct the deviation of the write end position due
to the error for each scanning optical system. When the image
forming device uses plural light sources, it is advantageous that
the initial value of the frequency can be set up separately, in
order to prevent the error for each scanning optical system. There
is known a method of detecting a write end position as disclosed in
Japanese Laid-Open Patent Publication No. 2004-102276. In this
method, the predetermined positioning marks are used as reference
marks when combining the images of the photoconductor drums 103 and
104, detected by the positioning sensors, and the amount of
deviation is computed based on the detection results of the
sensors. The initial value of the frequency is set up based on the
computation result of the amount of deviation.
Since the write end position may be varied over time, it is
preferred to set up the initial value of the frequency
appropriately.
[0188] Next, a second embodiment of the present disclosure will be
described. FIG. 30 is a diagram showing the composition of the
image forming device of the second embodiment of the present
disclosure.
[0189] As shown in FIG. 30, the image forming device 10 of the
second embodiment includes two light sources (LD 117, LD 127), two
photoconductor drums (103, 104), and one sync detecting sensor (PD
110).
[0190] Specifically, as shown in FIG. 30, the image forming device
10 includes a polygon mirror 100, scanning lenses 101 and 102,
photoconductor drums 103 and 104, incidence mirrors 105 and 106, a
PD (photodetector) 110, a pixel clock generating device 140, and
light sources (LD 117 and LD 127). The pixel clock generating
device 140 includes a first pixel clock generating part 141, a
false sync signal generating part 143, a second pixel clock
generating part 142, a third pixel clock generating part 144, a
first modulation data generating part 145, a second modulation data
generating part 147, a first laser driver 146, and a second laser
driver 148.
[0191] The laser incident light beam Bk from the light source 117
is reflected by the incidence mirror 105 to enter the polygon
mirror 100, and in sync with the rotation of the polygon mirror
100, passes through the scanning lens 101, so that the surface of
the photoconductor (Bk) 103 is scanned by the light beam.
[0192] On the other hand, the incident light beam Y from the light
source 127 is reflected by the incidence mirror 106 to enter the
polygon mirror 100, and in sync with the rotation of the polygon
mirror 100, passes through the scanning lens 102, so that the
surface of the light photoconductor (Y) 104 is scanned by the light
beam. Thereby, electrostatic latent images according to the outputs
of the light source 117 and the light source 127 are formed on the
photoconductor drum 103 and the photoconductor drum 104,
respectively.
[0193] The PD 110 is arranged at one end portion of the
photoconductor drum 103. The laser beam reflected by the polygon
mirror 100 enters the PD 110 before the scanning of a scanning line
by the laser beam is performed on the surface of the photoconductor
drum 103, and the timing of a start of the scanning is detected by
the PD 110. The timing of the start of the scanning detected by the
PD 110 is supplied to the pixel clock generating part-1 (141) of
the pixel clock generating device 140, and the false sync signal
generating part 143 as a first sync signal in sync with the
scanning of the photoconductor drum.
[0194] The pixel clock generating part-1 (141) generates a first
pixel clock and a frequency correction value based on the first
sync signal. The false sync signal generating part 143 generates a
false sync signal based on the first sync signal and the first
pixel clock. The pixel clock generating part-2 (142) corrects a
predetermined initial frequency setting value by the frequency
correction value, and generates a second pixel clock in sync with
the first sync signal. The pixel clock generating part-3 (144)
corrects a second predetermined initial frequency setting value
(which is separate from the initial frequency setting value set by
the pixel clock generating part-2 (142)) by the frequency
correction value, and generates a third pixel clock in sync with
the false sync signal.
[0195] Based on the first image data, the first modulation data
generating part 145 outputs first modulation data in sync with the
second pixel clock to the first laser driver 146. The first laser
driver 146 drives the light source 117 according to the first
modulation data, and the light source 117 emits a laser beam. Based
on the second image data, the second modulation data generating
part 147 outputs second modulation data in sync with the third
pixel clock to the second laser driver 148. The second laser driver
148 drives the light source 127 according to the second modulation
data, and the light source 127 emits a laser beam.
[0196] In the second embodiment, the polygon mirror having the four
faces is used, and the two of the faces of the polygon mirror are
used to perform the scanning of the two photoconductors as shown in
FIG. 30. However, the PD 110 is arranged only at one of the two
photoconductors, and the scanning of the photoconductor with which
no PD is arranged can be started in sync with the false sync signal
which is delayed by a predetermined time from the first sync signal
obtained from the PD 110.
[0197] FIG. 31 is a timing chart for operation of the false sync
signal generating part in the second embodiment. In the example of
FIG. 31, three counters (counters 0 to 2) are used to illustrate
the case where the two photoconductors are scanned by the two of
the faces in the polygon mirror.
[0198] As shown in FIG. 31, the first sync signal is asserted in
sync with the scanning of the photoconductor drum 103 at the time
of the scan start of the face-4 of the polygon mirror. The counters
0-2 are reset one by one by the first sync signal for the three
faces, and the counting is started by the first pixel clock.
[0199] If the counter 0 is reset for the face-1, the scan start
timing of the photoconductor drum 104 using the face-1 is the
timing set up by the false sync count number Nref_ps of the
comparator 1 after one or more of the faces of the polygon mirror
are rotated. The timing of the false sync signal is generated so
that the light source 127 writes on the photoconductor drum 104 for
the same face of the polygon mirror as the face of the polygon
mirror used when the light source 117 writes on the photoconductor
drum 103.
[0200] As described above, in the second embodiment, the frequency
of the first pixel clock is controlled according to the rotational
speed irregularity of the polygon mirror, a false sync signal is
generated based on the first pixel clock with the controlled
frequency, and the start position of the light beam on the opposite
side is fixed. The error for the faces of the polygon mirror can be
eliminated by generating the false sync signal. By setting up the
initial frequency for each pixel clock generating part, the error
of each scanning optical system can be corrected, and the error of
the write end position can be corrected.
[0201] The image modulation data is synchronized with the second
pixel clock based on the image data, and the image modulation data
is output as the light beam in the laser drive circuit.
[0202] Next, a third embodiment of the present disclosure will be
described. FIG. 32 is a diagram showing the composition of the
image forming device 10 of the third embodiment of the present
disclosure.
[0203] As shown in FIG. 32, the image forming device 10 of the
third embodiment includes one light source, two photoconductor
drums, and one sync detecting sensor (PD 110).
[0204] Apart from the second embodiment, in the image forming
device 10 of the third embodiment, the laser beam emitted from the
LD 117 is deflected by an optical beam division unit 107 to
generate incident light Y and incident light Bk. The function of
the pixel clock generating part-2 (142) to generate the second
pixel clock 124 and the function of the pixel clock generating
part-3 (144) to generate the third pixel clock 129 as shown in FIG.
30 are performed in this embodiment as follows. The pixel clock
generating part-4 (153) in the third embodiment of FIG. 32 changes
the frequency of the pixel clock 4 for one of the period the
surface of the photoconductor drum 103 is scanned and the period
the surface of the photoconductor drum 104 is scanned. The fifth
modulation data generating part 129 in the third embodiment of FIG.
32 is provided, instead of the first data modulation part 145 and
the second modulation data generating part 147 in the embodiment of
FIG. 30. The fifth laser driver (155) in the third embodiment of
FIG. 32 is provided, instead of the first laser driver (146) and
the second laser driver (148) in the embodiment of FIG. 30.
[0205] In the third embodiment of FIG. 32, the image forming device
10 includes a polygon mirror 100, two scanning lenses 101 and 102,
two photoconductor drums 103 and 104, two incidence mirrors 105 and
106, an optical beam division unit 107, a PD 110, a pixel clock
generating device 150, a modulation data generating part-5 (129), a
laser driver 5 (155), and a light source (LD) 117.
[0206] The pixel clock generating device 150 includes a pixel clock
generating part-1 (151), a false sync signal generating part 152,
and a pixel clock generating part-4 (153). The laser beam from the
light source 117 is divided into incident light Bk (the first light
beam) and incident light Y (the second light beam) by the optical
beam division unit 107.
[0207] The incident light Bk and the incident light Y enter the
face where they were reflected by the incidence mirrors 105 and
106, surfaces of the polygon mirrors 100, passing through the
scanning lenses 101 and 102, and scanned onto the photoconductor
(Bk) 103 and the photoconductor (Y) 104, respectively. Thereby, the
electrostatic latent images according to the output of the light
source 117 are formed on the photoconductor drums 103 and 104,
respectively.
[0208] The PD 110 is arranged at an end portion of the
photoconductor drum 103. The laser beam reflected by the polygon
mirror 100 enters the PD 110, before carrying out one-line scanning
of the photoconductor drum 103, and the timing of a start of the
scanning is detected by the PD 110.
[0209] The timing of the start of the scanning detected by the PD
110 is supplied to the pixel clock generating part-1 (151), false
sync signal generating part 152, and pixel clock generating part-4
(153) of pixel clock generating device 150 as the first periodic
sync signal united with the scanning of the photoconductor.
[0210] The pixel clock generating part-1 (151) generates pixel
clock 1 and a frequency correction value based on the first sync
signal. The false sync signal generating part 152 generates a false
sync signal based on the first sync signal and the pixel clock
1.
[0211] The pixel clock generating part-4 (153) generates the pixel
clock 4 by the frequency correction value of the first sync signal
and the false sync signal. Based on the image data, the modulation
data generating part-5 (129) generates the modulation data in sync
with pixel clock 4 and outputs the same to the laser driver 5
(155), so that the light source 117 is driven according to the
modulation data of the laser driver 5 (155) to output a laser
beam.
[0212] The scanning optical system of FIG. 32 is arranged to divide
the laser beam into incident light Bk and incident light Y using
the light beam division unit 107 which uses a half mirror.
[0213] FIG. 33 is a block diagram showing the composition of the
pixel clock generating part-4 (153). As shown in FIG. 33, the pixel
clock generating part-4 (153) includes an initial frequency setting
unit 601 which sets up the initial frequency of pixel clock 4 to
scan the photoconductor drum 103, an initial frequency setting unit
602 which sets up the initial frequency of pixel clock 4 to scan
the photoconductor drum 104, an adder 603, an OR circuit 605, a
digital control oscillator (DCO) 604, a selector 606, and a side
signal generating part 607.
[0214] The side signal generating part 607 detects which of the
photoconductor drums 103 and 104 is scanned, and changes a side
signal to H or L according to the first sync signal and the false
sync signal. The initial frequency setting unit 601 holds the first
initial frequency setting value, and the initial frequency setting
unit 602 holds the second initial frequency setting value.
[0215] The adder 603 adds the frequency correction value generated
by the first pixel clock generating part 112 and one of the initial
frequency setting values selected by the selector 606. In the phase
sync with the sync signal, the digital control oscillator (DCO) 604
generates the pixel clock 4.
[0216] FIG. 34 is a timing chart for explaining operation of the
pixel clock generating part-4 (153).
[0217] As shown in FIG. 34, the first sync signal is asserted when
the incident light Bk is detected by the PD 110. The false sync
signal is asserted in the above-mentioned timing after the first
sync signal is asserted. The side signal is changed to a high (H)
level if the false sync signal is asserted, and changed to a low
(K) level if the first sync signal is asserted. The sync signal is
the sum of the first sync signal and the false sync signal. The
first initial frequency setting value and the second initial
frequency setting value are stored in the register.
[0218] Ma denotes one of the setting value ma of the first initial
frequency and the setting value mb of the second initial frequency
which is selected by the side signal. When the side signal is at a
low (L) level, ma of the first initial frequency is selected, and
mb of the second initial frequency is selected when the side signal
is at a high (H) level.
[0219] Mi denotes a frequency setting value of the pixel clock
which is defined by the formula: Mi=(Ma+frequency correction value
.DELTA.f). Hence, the frequency setting value Mi of the pixel clock
is set to Mi=ma+.DELTA.f or Mi=mb+.DELTA.f in accordance with the
side signal. The frequency of the pixel clock is updated to
Mi=ma+.DELTA.f or Mi=mb+.DELTA.f by the sync signal, and in the
phase in sync with the sync signal, the digital control oscillator
604 outputs the pixel clock 4.
[0220] Because the pixel clock generating part-4 (153) and the
pixel clock generating part-1 (151) can be set up to different
initial frequency values, the setting of the initial frequency
value can be performed to eliminate the error of each scanning
optical system. According to the third embodiment, the initial
value of frequency can be set up to eliminate the error of each
scanning optical system, and it is possible to correct the error of
the write end positions.
[0221] As mentioned above, in the third embodiment, the write start
position of the light beam on the opposite side is fixed by
controlling the frequency of the pixel clock 1 according to the
rotational speed irregularity of the polygon mirror, and the false
sync signal is generated based on the pixel clock 1 with the
controlled frequency. The error of each face of the polygon mirror
can be disregarded by generating the false sync signal for the face
of the polygon mirror which is the same as that of the first sync
signal. By setting up plural initial frequencies individually with
a single LD, the error for each scanning optical system can be
corrected, and the error of the write end position can be
corrected.
[0222] Next, a fourth embodiment of the present disclosure will be
described. FIG. 35 is a diagram showing the composition of the
image forming device 10 of the fourth embodiment of the present
disclosure.
[0223] As shown in FIG. 35, the image forming device 10 of the
fourth embodiment includes four write laser diodes, four
photoconductor drums, and two sync detecting sensors.
[0224] When compared with the image forming device 10 of the second
embodiment shown in FIG. 30, the image forming device 10 shown in
FIG. 35 further includes photoconductor drums 703 and 704,
incidence mirrors 705 and 706, a PD 707, a third laser driver 1915,
a fourth laser driver 1909, and light sources 713 and 1901 which
are extended as the optical system. The pixel clock generating
device 1900 further includes a second false sync signal generating
part 1902, a clock generating part-5 (1910), a third modulation
data generating part 1913, a pixel clock generating part-6 (1904),
and a fourth modulation data generating part 1907.
[0225] The incident light M and the incident light Bk pass through
the scanning lens 101 in common, and the incident light Y and the
incident light C pass through the scanning lens 102 in common.
[0226] The pixel clock generating part-2 (142), the pixel clock
generating part-3 (144), the first modulation data generating part
145, the second modulation data generating part 147, the first
laser driver 146, and the second laser driver 148 in the fourth
embodiment are essentially the same as the pixel clock 2 (142), the
pixel clock generating part-3 (144), the first modulation data
generating part 145, the second modulation data generating part
147, the first laser driver 146, and the second laser driver 148 in
the second embodiment which have been described above with
reference to FIG. 30, and a description thereof will be
omitted.
[0227] The pixel clock generating part-5 (1910) and the sixth pixel
clock generating part (1904) in the fourth embodiment are
essentially the same as the pixel clock 2 (142) and the pixel clock
generating part-3 (144) in the second embodiment, and a description
thereof will be omitted. The third modulation data generating part
1913 and the fourth modulation data generating part 1907 in the
fourth embodiment are essentially the same as the first modulation
data generating part 145 and the second modulation data generating
part 147 in the second embodiment, and a description thereof will
be omitted. Further, the third laser driver 1915 and the fourth
laser driver 1909 in the fourth embodiment are essentially the same
composition as the first laser driver 146 and the second laser
driver 148 in the second embodiment, and a description thereof will
be omitted.
[0228] FIG. 36 is a timing chart for explaining operation of the
false sync signal generating part in the fourth embodiment.
[0229] When compared with the timing chart in the second embodiment
in FIG. 31, in the operation of the false sync signal generating
part shown in FIG. 36, with respect to the LD 117 and the LD 713,
the first sync signal from the PD 110 and the second sync signal
from the PD 707 are input to cause the comparators a0-a2 and b0-b2
and the counters a0-a2 and b0-b2 to operate so that the first false
sync signal and the second false sync signal are generated,
respectively. Other operations of the false sync signal generating
part in the fourth embodiment are essentially the same as in the
timing chart of FIG. 31, and a description thereof will be
omitted.
[0230] In the timing chart of FIG. 36, the timing when the false
sync signal is generated is the same as the timing the surface of
the photoconductor drum 104 is scanned by the light beam from the
same face of the polygon mirror 100 when the light beam from the LD
117 scans the surface of the photoconductor drum 103. The timing
when the second false sync signal is generated is the same as the
timing the surface of the photoconductor drum 704 is scanned by the
light beam from the same face of the polygon mirror 100 when the
light beam from the LD 713 scans the surface of the photoconductor
drum 703.
[0231] In the fourth embodiment, the frequency of the first pixel
clock is controlled according to the rotational speed irregularity
of the polygon mirror, and a false sync signal is generated based
on the controlled frequency of the first pixel clock, so that the
write start position of the light beam for the opposite side of the
opposed scanning sides is fixed.
[0232] In the fourth embodiment, the first false sync signal is
generated for the same face as that of the first sync signal, and
the errors for the respective faces of the polygon mirror can be
disregarded. Similarly, the second false sync signal is generated
for the same face as that of the second sync signal, and the errors
for the respective faces of the polygon mirror can be disregarded.
The setting of the initial frequency is performed for each LD, and
the error for each scanning optical system can be corrected. Even
if the number of photoconductor drums is increased to be larger
than that in the second embodiment, the error of the write end
position can be corrected.
[0233] Next, a timing chart in which a sync signal detected by the
PD 707 is divided into a first sync signal and a second sync signal
will be described with reference to FIG. 37. FIG. 37 is a timing
chart in which the first sync signal and the second sync signal are
separated from the sync signal.
[0234] As shown in FIG. 37, the sync signal detected by the PD 707
is allocated to either the first sync signal or the second sync
signal in accordance with a sync select signal obtained from the
CPU or the control unit of the host device. In the example of FIG.
37, when the sync select signal is at a low level L, the sync
signal is separated to the first sync signal, and when the sync
select signal is at a high level H, the sync signal is separated to
the second sync signal.
[0235] With the use of the sync select signal obtained from the
control unit of the host device or the CPU, the sync signal can be
separated into the first sync signal and the second sync signal
even if a single PD is used in common.
[0236] Next, a fifth embodiment of the present disclosure will be
described. FIG. 38 is a diagram showing of the composition of the
image forming device 10 of the fifth embodiment of the present
disclosure.
[0237] As shown in FIG. 38, the image forming device 10 of the
fifth embodiment includes two light sources, four photoconductor
drums, and two sync detecting sensors.
[0238] When compared with the image forming device of the third
embodiment in FIG. 32, the image forming device 10 shown in FIG. 38
further includes photoconductor drums 703 and 704, incidence
mirrors 705 and 706, a PD 707, a laser driver 712, a light source
713, and a modulation data generating part 711 that outputs the
second modulation data to the laser driver 712.
[0239] The LD 713 which is disposed at a position distant from the
LD 117 is illustrated in FIG. 38. However, in the actual optical
system, the LD 713 may be disposed at the position distant from the
LD 117 in the sub-scanning direction. The incident light M and the
incident light Bk pass through the scanning lens 101 in common, and
the incident light Y and the incident light C pass through the
scanning lens 102 in common.
[0240] In the fifth embodiment, the same operation as the third
embodiment is performed. However, in the fifth embodiment, the
setting of an initial frequency is performed for each of the pixel
clock generating part-4 (128) and the pixel clock generating part-7
(710), and the error for each scanning optical system can be
corrected.
[0241] The pixel clock generating part-4 (128) and the pixel clock
generating part-7 (710) have the same composition as the pixel
clock generating part-4 (131) shown in FIG. 33, and a description
thereof will be omitted. The pixel clock generating part-4 (128)
generates a pixel clock 4 which is a reference clock of the
incident light Bk and the incident light Y, and the pixel clock
generating part-7 (710) generates a seventh pixel clock which is a
reference clock of the incident light M and the incident light
C.
[0242] The polygon mirror 100 can be shared by the pixel clock
generating part-4 (128) and the pixel clock generating part-7
(710), and rotation irregularity of each pixel clock generating
part is equal. Hence, the pixel clock generating part-1 (112) can
be used with these pixel clock generating parts, and it is possible
to provide a simple structure of the circuit.
[0243] The operation of the false sync signal generating part in
the fifth embodiment is essentially the same as the operation in
the timing chart of FIG. 36 regarding the fourth embodiment, and a
description thereof will be omitted.
[0244] Next, a sixth embodiment of the present disclosure will be
described. FIG. 39 is a diagram showing the composition of an image
forming device of the sixth embodiment of the present
disclosure.
[0245] In the image forming device in FIG. 39, the PD 110 in the
image forming device of the fifth embodiment shown in FIG. 38 is
omitted, only the PD 707 remains, and a selector 1001 is
installed.
[0246] As shown in FIG. 39, the first sync signal from the PD 707
is shared by the false sync signal generating part 709 and the
pixel clock generating part-7 (710). Although the single PD 707 is
used, the write timing of the LD 117 and the write timing of the LD
713 differ from each other, and the false sync signal generating
part uses the false sync signal generating part 709 and the second
false sync signal generating part 1902, similar to the fifth
embodiment.
[0247] The LD 713 which is disposed at a position distant from the
LD 117 is illustrated in FIG. 39. However, in the actual optical
system, the LD 713 may be disposed in the vicinity of the LD
117.
[0248] In the sixth embodiment, the same operation as the fifth
embodiment is performed. However, the setting of an initial
frequency is performed for each of the pixel clock generating
part-4 (128) and the pixel clock generating part-7 (710), and the
error for each scanning optical system can be corrected.
[0249] The pixel clock generating part-4 (128) and the pixel clock
generating part-7 (710) have the same composition as the pixel
clock generating part-4 (131) shown in FIG. 33, and a description
thereof will be omitted. The pixel clock generating part-4 (128)
generates a pixel clock 4 which is a reference clock of the
incident light Bk and the incident light Y. The pixel clock
generating part-7 (710) generates a seventh pixel clock which is a
reference clock of the incident light M and the incident light C.
The polygon mirror 100 is shared by the pixel clock generating
part-4 (128) and the pixel clock generating part-7 (710), and
rotation irregularities of the pixel clock generating parts are
equal. Hence, the first pixel clock generating part 112 can be used
with these pixel clock generating parts, and it is possible to
provide a simple structure of the circuit.
[0250] FIG. 40 is a timing chart for explaining operation of the
false sync signal generating part in the sixth embodiment. The
operation of the false sync signal generating part shown in FIG. 40
is essentially the same as shown in FIG. 36, and the first sync
signal from the PD 110 is used to cause the comparators a0-a2 and
b0-b2 and the counters a0-a2 and b0-b2 to operate so that the first
false signal and the second false signal are generated.
[0251] According to the optical scanning apparatus of the present
disclosure, even when a sync signal generating unit (PD) is
arranged only on one side of the opposed scanning sides, based on
the sync signal of the sync signal generating unit, a false sync
signal for the opposite side of the opposed scanning sides is
generated with high precision. Therefore, the above-mentioned
errors are corrected and the write start position and the write end
position can be corrected with high precision.
[0252] The present disclosure is not limited to the specifically
disclosed embodiments, and variations and modifications may be made
without departing from the scope of the present disclosure.
[0253] The present application is based upon and claims the benefit
of priority of Japanese Patent Application No. 2011-277161, filed
on Dec. 19, 2011, Japanese Patent Application No. 2012-023572,
filed on Feb. 7, 2012, and Japanese Patent Application No.
2012-265706, filed on Dec. 4, 2012, the contents of which are
incorporated herein by reference in their entirety.
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